Simple Variable Attack/Release for THAT4301 Compressor

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mediatechnology
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Simple Variable Attack/Release for THAT4301 Compressor

Post by mediatechnology » Fri Nov 16, 2018 12:32 pm

Note that U5A is connected as a comparator and that C3 is referenced to -15V so that it can remain polarized.

Image
Variable Slew Limiter Circuit from http://burnit.co.uk/sdiy/images/boxotri ... ressor.gif

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JR.
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Re: Simple Variable Attack/Release for THAT4301 Compressor

Post by JR. » Fri Nov 16, 2018 1:49 pm

Using steering diodes is common practice for att/release circuits. Embedding this inside a feedback loop overcomes the directional diode drop threshold but not the +/- diode drop dithering around equilibrium. I can imagine different improvements but never found linear slopes*** very useful in dynamics side chains.

JR

*** one exception was when I used a DBX VCA on a console master fader for auto fades (circa 1980s)..in that one case dB/sec decay was my goal, so I made it linear.

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mediatechnology
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Re: Simple Variable Attack/Release for THAT4301 Compressor

Post by mediatechnology » Fri Nov 16, 2018 3:15 pm

I recall the original application for this was Electronic Music.
This particular one has been pretty widely-used in a kit IIRC.

The slopes in the dBx adjustable attack-release circuit were also linear being current sources/sinks charging/discharging a capacitor.
FWIW the 4301 is linear dB/V.

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JR.
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Re: Simple Variable Attack/Release for THAT4301 Compressor

Post by JR. » Fri Nov 16, 2018 4:37 pm

Linear or log gain law, in dynamics side chains it is often about managing little and big dynamic steps. RC time constants naturally slow down as the difference gets smaller.

If using linear slew is desired I would at least put a small cap across the first op amp to slow down all the dithering.. perhaps a large resistor in parallel with the steering diodes again to reduce dithering.

JR

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mediatechnology
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Re: Simple Variable Attack/Release for THAT4301 Compressor

Post by mediatechnology » Fri Nov 16, 2018 5:31 pm

It's being used as a comparator so some positive feedback around U5A might help too.

Notice that he characterizes it as "linear enough." :lol:

The THAT version was a bear to stabilize: http://www.thatcorp.com/datashts/dn115.pdf

Image
THAT DN-115 Adjustable Attack Release Schematic

Note U1A is an open-collector output comparator.

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JR.
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Re: Simple Variable Attack/Release for THAT4301 Compressor

Post by JR. » Thu Nov 22, 2018 10:51 am

Yup I recall seeing that... didn't look like a useful circuit to manufacture (too many discrete parts) and the current mirrors don't look very stable over temperature. Might be a good app for an OTA (like old 3080).

JR

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